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<title>Exuberant Ctags: Adding support for a new language</title>
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<h1>How to Add Support for a New Language to Exuberant Ctags</h1>

<p>
<b>Exuberant Ctags</b> has been designed to make it very easy to add your own
custom language parser. As an exercise, let us assume that I want to add
support for my new language, <em>Swine</em>, the successor to Perl (i.e. Perl
before Swine &lt;wince&gt;). This language consists of simple definitions of
labels in the form "<code>def my_label</code>". Let us now examine the various
ways to do this.
</p>

<h2>Operational background</h2>

<p>
As ctags considers each file name, it tries to determine the language of the
file by applying the following three tests in order: if the file extension has
been mapped to a language, if the file name matches a shell pattern mapped to
a language, and finally if the file is executable and its first line specifies
an interpreter using the Unix-style "#!" specification (if supported on the
platform). If a language was identified, the file is opened and then the
appropriate language parser is called to operate on the currently open file.
The parser parses through the file and whenever it finds some interesting
token, calls a function to define a tag entry.
</p>

<h2>Creating a user-defined language</h2>

<p>
The quickest and easiest way to do this is by defining a new language using
the program options. In order to have Swine support available every time I
start ctags, I will place the following lines into the file
<code>$HOME/.ctags</code>, which is read in every time ctags starts:

<code>
<pre>
  --langdef=swine
  --langmap=swine:.swn
  --regex-swine=/^def[ \t]*([a-zA-Z0-9_]+)/\1/d,definition/
</pre>
</code>
The first line defines the new language, the second maps a file extension to
it, and the third defines a regular expression to identify a language
definition and generate a tag file entry for it.
</p>

<h2>Integrating a new language parser</h2>

<p>
Now suppose that I want to truly integrate compiled-in support for Swine into
ctags. First, I create a new module, <code>swine.c</code>, and add one
externally visible function to it, <code>extern parserDefinition
*SwineParser(void)</code>, and add its name to the table in
<code>parsers.h</code>. The job of this parser definition function is to
create an instance of the <code>parserDefinition</code> structure (using
<code>parserNew()</code>) and populate it with information defining how files
of this language are recognized, what kinds of tags it can locate, and the
function used to invoke the parser on the currently open file.
</p>

<p>
The structure <code>parserDefinition</code> allows assignment of the following
fields:

<code>
<pre>
  const char *name;               /* name of language */
  kindOption *kinds;              /* tag kinds handled by parser */
  unsigned int kindCount;         /* size of `kinds' list */
  const char *const *extensions;  /* list of default extensions */
  const char *const *patterns;    /* list of default file name patterns */
  parserInitialize initialize;    /* initialization routine, if needed */
  simpleParser parser;            /* simple parser (common case) */
  rescanParser parser2;           /* rescanning parser (unusual case) */
  boolean regex;                  /* is this a regex parser? */
</pre>
</code>
</p>

<p>
The <code>name</code> field must be set to a non-empty string. Also, unless
<code>regex</code> is set true (see below), either <code>parser</code> or
<code>parser2</code> must set to point to a parsing routine which will
generate the tag entries. All other fields are optional.

<p>
Now all that is left is to implement the parser. In order to do its job, the
parser should read the file stream using using one of the two I/O interfaces:
either the character-oriented <code>fileGetc()</code>, or the line-oriented
<code>fileReadLine()</code>. When using <code>fileGetc()</code>, the parser
can put back a character using <code>fileUngetc()</code>. How our Swine parser
actually parses the contents of the file is entirely up to the writer of the
parser--it can be as crude or elegant as desired. You will note a variety of
examples from the most complex (c.c) to the simplest (make.c).
</p>

<p>
When the Swine parser identifies an interesting token for which it wants to
add a tag to the tag file, it should create a <code>tagEntryInfo</code>
structure and initialize it by calling <code>initTagEntry()</code>, which
initializes defaults and fills information about the current line number and
the file position of the beginning of the line. After filling in information
defining the current entry (and possibly overriding the file position or other
defaults), the parser passes this structure to <code>makeTagEntry()</code>.
</p>

<p>
Instead of writing a character-oriented parser, it may be possible to specify
regular expressions which define the tags. In this case, instead of defining a
parsing function, <code>SwineParser()</code>, sets <code>regex</code> to true,
and points <code>initialize</code> to a function which calls
<code>addTagRegex()</code> to install the regular expressions which define its
tags. The regular expressions thus installed are compared against each line 
of the input file and generate a specified tag when matched. It is usually
much easier to write a regex-based parser, although they can be slower (one
parser example was 4 times slower). Whether the speed difference matters to
you depends upon how much code you have to parse. It is probably a good
strategy to implement a regex-based parser first, and if it is too slow for
you, then invest the time and effort to write a character-based parser.
</p>

<p>
A regex-based parser is inherently line-oriented (i.e. the entire tag must be
recognizable from looking at a single line) and context-insensitive (i.e the
generation of the tag is entirely based upon when the regular expression
matches a single line). However, a regex-based callback mechanism is also
available, installed via the function <code>addCallbackRegex()</code>. This
allows a specified function to be invoked whenever a specific regular
expression is matched. This allows a character-oriented parser to operate
based upon context of what happened on a previous line (e.g. the start or end
of a multi-line comment). Note that regex callbacks are called just before the
first character of that line can is read via either <code>fileGetc()</code> or
using <code>fileGetc()</code>. The effect of this is that before either of
these routines return, a callback routine may be invoked because the line
matched a regex callback. A callback function to be installed is defined by
these types:

<code>
<pre>
  typedef void (*regexCallback) (const char *line, const regexMatch *matches, unsigned int count);

  typedef struct {
      size_t start;   /* character index in line where match starts */
      size_t length;  /* length of match */
  } regexMatch;
</pre>
</code>
</p>

<p>
The callback function is passed the line matching the regular expression and
an array of <code>count</code> structures defining the subexpression matches
of the regular expression, starting from \0 (the entire line).
</p>

<p>
Lastly, be sure to add your the name of the file containing your parser (e.g.
swine.c) to the macro <code>SOURCES</code> in the file <code>source.mak</code>
and an entry for the object file to the macro <code>OBJECTS</code> in the same
file, so that your new module will be compiled into the program.
</p>

<p>
In case you have some problems run <code>ctags --verbose</code> to see if the
extensions or patterns you defined for your language conflict with other
languages.
</p>

<p>
This is all there is to it. All other details are specific to the parser and
how it wants to do its job. There are some support functions which can take
care of some commonly needed parsing tasks, such as keyword table lookups (see
keyword.c), which you can make use of if desired (examples of its use can be
found in c.c, eiffel.c, and fortran.c). Almost everything is already taken care
of automatically for you by the infrastructure.  Writing the actual parsing
algorithm is the hardest part, but is not constrained by any need to conform
to anything in ctags other than that mentioned above.
</p>

<p>
There are several different approaches used in the parsers inside <b>Exuberant
Ctags</b> and you can browse through these as examples of how to go about
creating your own.
</p>

<h2>Examples</h2>

<p>
Below you will find several example parsers demonstrating most of the
facilities available. These include three alternative implementations
of a Swine parser, which generate tags for lines beginning with
"<code>def</code>" followed by some name.
</p>

<code>
<pre>
/***************************************************************************
 * swine.c
 * Character-based parser for Swine definitions
 **************************************************************************/
/* INCLUDE FILES */
#include "general.h"    /* always include first */

#include &lt;string.h&gt;     /* to declare strxxx() functions */
#include &lt;ctype.h&gt;      /* to define isxxx() macros */

#include "parse.h"      /* always include */
#include "read.h"       /* to define file fileReadLine() */

/* DATA DEFINITIONS */
typedef enum eSwineKinds {
    K_DEFINE
} swineKind;

static kindOption SwineKinds [] = {
    { TRUE, 'd', "definition", "pig definition" }
};

/* FUNCTION DEFINITIONS */

static void findSwineTags (void)
{
    vString *name = vStringNew ();
    const unsigned char *line;

    while ((line = fileReadLine ()) != NULL)
    {
        /* Look for a line beginning with "def" followed by name */
        if (strncmp ((const char*) line, "def", (size_t) 3) == 0  &amp;&amp;
            isspace ((int) line [3]))
        {
            const unsigned char *cp = line + 4;
            while (isspace ((int) *cp))
                ++cp;
            while (isalnum ((int) *cp)  ||  *cp == '_')
            {
                vStringPut (name, (int) *cp);
                ++cp;
            }
            makeSimpleTag (name, SwineKinds, K_DEFINE);
            vStringClear (name);
        }
    }
    vStringDelete (name);
}

/* Create parser definition stucture */
extern parserDefinition* SwineParser (void)
{
    static const char *const extensions [] = { "swn", NULL };
    parserDefinition* def = parserNew ("Swine");
    def-&gt;kinds      = SwineKinds;
    def-&gt;kindCount  = KIND_COUNT (SwineKinds);
    def-&gt;extensions = extensions;
    def-&gt;parser     = findSwineTags;
    return def;
}
</pre>
</code>

<p>
<pre>
<code>
/***************************************************************************
 * swine.c
 * Regex-based parser for Swine
 **************************************************************************/
/* INCLUDE FILES */
#include "general.h"    /* always include first */
#include "parse.h"      /* always include */

/* FUNCTION DEFINITIONS */

static void installSwineRegex (const langType language)
{
    addTagRegex (language, "^def[ \t]*([a-zA-Z0-9_]+)", "\\1", "d,definition", NULL);
}

/* Create parser definition stucture */
extern parserDefinition* SwineParser (void)
{
    static const char *const extensions [] = { "swn", NULL };
    parserDefinition* def = parserNew ("Swine");
    def-&gt;patterns   = patterns;
    def-&gt;extensions = extensions;
    def-&gt;initialize = installSwineRegex;
    def-&gt;regex      = TRUE;
    return def;
}
</code>
</pre>

<p>
<pre>
/***************************************************************************
 * swine.c
 * Regex callback-based parser for Swine definitions
 **************************************************************************/
/* INCLUDE FILES */
#include "general.h"    /* always include first */

#include "parse.h"      /* always include */
#include "read.h"       /* to define file fileReadLine() */

/* DATA DEFINITIONS */
typedef enum eSwineKinds {
    K_DEFINE
} swineKind;

static kindOption SwineKinds [] = {
    { TRUE, 'd', "definition", "pig definition" }
};

/* FUNCTION DEFINITIONS */

static void definition (const char *const line, const regexMatch *const matches,
                       const unsigned int count)
{
    if (count &gt; 1)    /* should always be true per regex */
    {
        vString *const name = vStringNew ();
        vStringNCopyS (name, line + matches [1].start, matches [1].length);
        makeSimpleTag (name, SwineKinds, K_DEFINE);
    }
}

static void findSwineTags (void)
{
    while (fileReadLine () != NULL)
        ;  /* don't need to do anything here since callback is sufficient */
}

static void installSwine (const langType language)
{
    addCallbackRegex (language, "^def[ \t]+([a-zA-Z0-9_]+)", NULL, definition);
}

/* Create parser definition stucture */
extern parserDefinition* SwineParser (void)
{
    static const char *const extensions [] = { "swn", NULL };
    parserDefinition* def = parserNew ("Swine");
    def-&gt;kinds      = SwineKinds;
    def-&gt;kindCount  = COUNT_ARRAY (SwineKinds);
    def-&gt;extensions = extensions;
    def-&gt;parser     = findSwineTags;
    def-&gt;initialize = installSwine;
    return def;
}
</pre>

<p>
<pre>
/***************************************************************************
 * make.c
 * Regex-based parser for makefile macros
 **************************************************************************/
/* INCLUDE FILES */
#include "general.h"    /* always include first */
#include "parse.h"      /* always include */

/* FUNCTION DEFINITIONS */

static void installMakefileRegex (const langType language)
{
    addTagRegex (language, "(^|[ \t])([A-Z0-9_]+)[ \t]*:?=", "\\2", "m,macro", "i");
}

/* Create parser definition stucture */
extern parserDefinition* MakefileParser (void)
{
    static const char *const patterns [] = { "[Mm]akefile", NULL };
    static const char *const extensions [] = { "mak", NULL };
    parserDefinition* const def = parserNew ("Makefile");
    def-&gt;patterns   = patterns;
    def-&gt;extensions = extensions;
    def-&gt;initialize = installMakefileRegex;
    def-&gt;regex      = TRUE;
    return def;
}
</pre>

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